Mark “Murch” Erhardt and Mike Schmidt are joined by Sergi Delgado Segura to discuss Newsletter #269.

The Bitcoin Optech Podcast and transcription content is licensed Creative Commons CC BY-SA 2.0


Changes to services and client software

  • Bitcoin-like Script Symbolic Tracer (B'SST) released (9:23)

  • STARK header chain verifier demo (10:53)

  • JoinMarket v0.9.10 released (13:07)

  • BitBox adds miniscript (20:18)

  • Machankura announces additive batching feature (13:51)

  • SimLN Lightning simulation tool (16:47)

Releases and release candidates

Notable code and documentation changes


Mike Schmidt: Welcome everyone to Bitcoin Optech Recap #269 on Twitter Spaces. It’s Thursday, September 21, and we are joined this week by Sergi Delgado to talk about Bitcoin Research Day event, and we also highlight some interesting software updates in our monthly services and client software segment. We’ll go through some introductions and then jump in. I’m Mike Schmidt, contributor at Bitcoin Optech and Executive Director at Brink, funding Bitcoin open-source developers. Murch?

Mark Erhardt: Hi, I’m Murch, I work at Chaincode Labs. I’m currently Wi-Fi-challenged!

Mike Schmidt: Sergi?

Sergi Delgado: Hello, I’m Sergi, I also work at Chaincode now and my Wi-Fi is not great either! And thank you for having me today here to talk about the Bitcoin Research Day.

Bitcoin research event

Mike Schmidt: Well, the Bitcoin Research event that we referenced just a second ago is actually the first news item, so we’ll jump into that. Sergi, do you want to introduce maybe the what of what is this Research Day, and maybe we can get into why you guys are putting that on, and maybe some of the content that will be there?

Sergi Delgado: Sure. So what this is, is a one-day event that is going to happen later this year in October in the Chaincode office. And the idea of it is kind of bringing together researchers from the Bitcoin space, and developers, to talk about the things that make Bitcoin what it is; make it robust, make it secure, make it somehow private. And it’s kind of a way of bringing them back together. Because it feels like at some point, the research community and the development community were quite tied together, but that feels like it’s not the case anymore. There’s research being done on Bitcoin, and there’s a lot of development being done in Bitcoin, but it feels like the two sides are not collaborating enough or not listening enough to each other. So that feels it could be improved, so that’s kind of the rationale for why we are doing this.

Mike Schmidt: So, what kind of discussions are scheduled to be had, or do you hope will be had with, I saw that there’s speakers and Lightning talks; maybe you can get into the topics?

Sergi Delgado: So, the idea of how the setup is going to look like, there’s going to be long-format talks from both researchers and developers covering topics from cryptography to P2P, from BIP writing to actually implementing the BIP; there’s consensus; there’s what else? I think those are currently the main topics. I would need to go over the website to remember a little bit more. But those are going to be long forms. Whoever is in charge of that talk will give the presentation. There’s going to be discussion around it after that.

But we also are encouraging people who are working in the space, and they may have early-stage research, or they are working on a specific development project, that want to give a short talk, like a much more lightning talk, or a five-minute talk, to come around and talk about what they’re doing. That may kickstart a collaboration program later on with either some of the local developers or some of the researchers. That’s about it for the speaking slots.

But what we also want to have are people who are interested in this. You don’t need to be a researcher or a developer, but we encourage you to be, so the discussion can be open to more than just the speakers.

Mike Schmidt: So, you mentioned you’re targeting a certain audience to attend. So, this is probably a curious topic for listeners here, but you’re encouraging an ideal attendee to be an academic researcher or a developer in order to sign up to attend; is that right?

Sergi Delgado: Sure. I mean, it’s not like you’re required to be, but the focus of the day is going to be really technical, like the whole point of it is fostering collaboration. So, if you are not, you’re also welcome to come, but you may have less to say, or you may get more bored by what is going to end up happening, right? We are targeting this to be a highly technical day with a purpose in mind.

Mike Schmidt: And is this something that’s going to be recorded, audio, video, or like a transcript of it, or is that more meant to be sort of an internal, in-person meeting only?

Sergi Delgado: So, it’s in-person, that’s for sure. I don’t think we’re going to be streaming it. There may be some transcripts that we haven’t decided on yet. But it’s mainly going to be something to be had in person. So, again, there’s no tickets, you don’t have to buy anything to come, you just have to go to the website and sign up. There’s a form for that, both for the Lightning talks and the attendee list. We cannot ensure that that’s going to be transcripted. If we have the means to do it, we may do our best to do it, but in the same sense that when we run the Socratic seminars, we want to encourage people to participate and to say their piece; we don’t want to disencourage people to say what they feel they have to say. So, if that gets in the way of people speaking up, then we won’t do it. So, if that’s not the case, if people are happy with it and we have the means to do it, we may do it. But I guess no promises here.

Mike Schmidt: And I see a few great speakers listed on the website. The website, by the way, is One of the speakers is our very own Murch. Murch, do you have a topic that you’d like to share that you’ll be talking about?

Mark Erhardt: I think that I might be getting a bit into how I started as a researcher writing my master thesis about coin selection, and how that translated into developers putting that into various code bases, and sort of as an example of how this collaboration will go or could go.

Mike Schmidt: So, if you’re a developer or a researcher, head to the website,, and there’s a big yellow “Sign me up!” link, and hopefully you can collaborate with some smart people. Sergi, any parting words of encouragement for our audience?

Sergi Delgado: Not much more. I would say I think it’s going to be a really interesting event. If you are somehow interested in research or developing Bitcoin, I think it’s something you shouldn’t miss out on. It’s going to be in Chaincode, you know, it’s one of the most special locations you can have in New York, so it’s going to be worth coming. There will be a happy hour later where you could also discuss more general topics. There’s going to be the – oh, I completely forgot about that, that’s true. There’s the Bitcoin Research Prize Ceremony in where – so let me phrase this properly.

Some time ago, there was a process for submission of academic papers within 2019 to 2023, I think it was, or 2018 to 2023, I don’t remember the exact timeframe, to decide what was the best Bitcoin-related research paper within that time period. Then a committee was selected to vote on that, and the announcement of the winner would be had during the research day. And they are also going to be talking about the paper itself, so they will make a presentation about it. That’s something that we are exploring. We don’t know if that’s going to happen again or not, but it feels like it’s also a good way of encouraging people to do research in things that matter.

Mike Schmidt: Excellent. Well, thanks, Sergi, for coming on and promoting this. I think it’s a good opportunity for certain folks in the community to collaborate, and I’m glad you guys are doing that. So, thanks for joining us. You’re welcome to stay on, but you’re welcome to drop as well if you have other things you’re doing.

Sergi Delgado: No worries. Thank you guys for having me.

Mike Schmidt: Next segment of the newsletter is our monthly segment on client and service software updates, things that we think are interesting developments or adoption of Bitcoin tech that we want to surface for the community.

Bitcoin-like Script Symbolic Tracer (B’SST) released

The first one here is Bitcoin-like Script Symbolic Tracer, which is acronymed as B’SST. And this is a tool that, “Symbolically executes opcodes, tracks constraints that opcodes impose on values they operate on, and shows the report with conditions that the script enforces, possible failures, or possible values for data, etc”. So, you feed it either a Bitcoin script or the element script, which is the basis of the Liquid sidechain, and you can provide those scripts to this tool, which is written in Python, and it will analyze that script and provide you some interesting report with those different conditions and possible failures. Murch, did you look at B’SST?

Mark Erhardt: Just very briefly. I think it’s probably interesting in the context of the ways that people might consider making output script descriptors in the future. It’s maybe also interesting in the discussion of what sort of new opcodes we want to introduce for introspection or covenants, because a bunch of those are, for example, already in elements. So, that’s just the ideas that come to mind when I read about this, but I haven’t looked too much at it.

STARK header chain verifier demo

Mike Schmidt: The next piece of software that we highlighted was a demo that was put together by the ZeroSync project. We covered a ZeroSync item in Newsletter #222, if you’re curious. We spoke about ZeroSync and their project of using utreexo and STARK proofs to sync a Bitcoin node, like you would do in Initial Block Download (IBD). And this demo that they have that we highlighted this week is a demonstration of using STARKs to prove and verify a chain of Bitcoin block headers. So, they’re just proving the chain of headers, they’re not doing the whole blocks yet. I think there’s a lot of resource intensity that goes into that, so they’re starting with just the chain of Bitcoin block headers. So, I thought that was interesting. There’s a lot of talk about STARKs and these sorts of proofs lately, so that was a cool demo. So, check it out if you’re curious about some of this syncing that they’re working on.

Mark Erhardt: Yeah, one of the interesting applications here would be, of course, if you can prove that you have processed correctly the entire header chain, and that potentially then in the future you have also processed the entire blockchain correctly, and you can tie that to a UTXO set, for example; that would be another way of how you could very quickly bootstrap a full node to the current state of the network.

Mike Schmidt: It seems like there’s a lot of different projects working on things like this to get somebody up and running with a node that is eventually fully validating, but there’s maybe some shortcuts in between. So, good to see.

Mark Erhardt: Yeah, I think this one actually would also work very well to make light clients less trustful. So, if you can prove that you have correctly processed the header chain, and then you can prove that a transaction is committed to by that header chain, then that would be a completely different light client model.

JoinMarket v0.9.10 released

Mike Schmidt: Next piece of software is JoinMarket. In their v0.9.10 release, they added support for RBF. JoinMarket is a coinjoin software and the RBF support that they’ve added is not relevant to the coinjoin transactions and fee bumping, but non-coinjoin transactions can now be fee bumped. And this release also included some fee estimation updates and some other improvements as well.

Machankura announces additive batching feature

I will probably butcher the name of this next piece of software, so I apologize in advance to the team, but Machankura announces additive batching feature, and so this is an interesting site. I think they provide a lot of Bitcoin-related services in areas where maybe there’s not a lot of cell coverage, or there is cell coverage, but maybe not a lot of Wi-Fi, people are working on more traditional phones. You can interact with Bitcoin using sort of a flip phone kind of thing. And so they have the ability to buy and sell, I believe, and transact. I think there’s also a web interface for their service as well.

What we talked about that they announced on Twitter recently was a beta feature that supports additive batching using RBF, and they’re doing that in a taproot wallet that has a FROST threshold spending condition. So, a lot of good Bitcoin tech in there, in their beta feature. If you look at the tweet thread in which they announced this, they gave an example of the additive batching, which the example was, there was an initial user that wanted to withdraw 21,000 sats and so they showed a screenshot on of that initial transaction, and then they then fee bumped using RBF and added an additional withdrawal, potentially for a different user, for 22,000 sats. And you see that UTXO being added, and then a third one for 23,000 sats. Murch, any comments on additive batching or my explanation of additive batching using RBF?

Mark Erhardt: I think it’s pretty cool that people are building this sort of stuff. I think, sure, RBF came out in 2013, but there’s really only after this block space market that we have been seeing this year, I think more people are exploring how they can leverage RBF to improve their operations. And so, CPFP has been adopted way more broadly, but of course CPFP comes with the downside of needing to have another transaction and thus buying more block space when you want to bump an existing transaction. So, with the RBF batching, you just add the output to an existing transaction. You also increase the fees, so you may have trade-offs at some points. It might become better to CPFP again instead of RBFing or starting a new additive batch, but yeah, I think this is just a sign of people making more use of the tools that they have because the market conditions have changed.

SimLN Lightning simulation tool

Mike Schmidt: Necessity is the mother of invention, right? The last piece of software that we highlighted this week was SimLN, which is a Lightning simulation tool. We spoke about a similar effort maybe a month ago, which is called Scaling Lightning, and they were, I think, trying to figure out a way to spin up Kubernetes with a bunch of different Lightning implementations and maybe even different versions with different things supported, and using that as a developer tool to test changes in those different implementations, by executing a bunch of different scenarios and then being able to do that in a collaborative way.

But SimLN is a Rust simulation tool that generates realistic Lightning payment activity. And in this initial release that we covered, it supports LND and Core Lightning (CLN) implementations, but there’s also work being done already on Eclair and the LDK Node node as well. And in the description of the project, I saw, “It is intentionally environment-agnostic so that it can be used across many environments, from integration tests to public signets”. And so, this may be a useful tool if you are one of the following: a protocol developer looking to test proposals, an application developer load-testing your application, a signet operator interested in a hands-off way to run an active node, or a researcher generating synthetic data for a target topology. And those were four different use cases that I read from the writeup on GitHub. Murch, are you familiar with the SimLN tool?

Mark Erhardt: Yeah, I heard someone talk about it recently. So, some of my colleagues worked on this. The interesting thing here is also that you can do some live compatibility testing here, right? So, by having a simulated network that is not just one type of implementation, you could, for example, have old versions and new versions of the same node software. You could have different node software interacting, and you can automate activities. So, for example, one user might send a transaction at a specific interval for a specific amount, some other nodes might randomly send or not send at specific intervals, the amounts might change.

So, you’ll naturally get into situations where maybe some channels are exhausted in one direction but not in the other. And you might just be able to see, do all the nodes properly resolve this sort of behavior with each other, even across different software bases. You might be able to sort of test the jamming mitigation proposals that have been worked on for quite some time. That sort of thing is the background here.

Mike Schmidt: Great examples, Murch, thanks for clarifying and adding to that. Next segment of the newsletter is Releases and release candidates. We have two –

Mark Erhardt: I think we omitted BitBox. How about we do that stuff?

BitBox adds miniscript

Mike Schmidt: Oh, I did. Thank you for catching that. Back to the client and services section. BitBox adds miniscript. So, the Bitbox02 firmware adds support for miniscript and security fix and some usability improvements. So, the Bitbox02 is a hardware signing device, and with the proliferation of miniscript getting a lot of uptake lately, they’ve added support for miniscript. So, now you can sign with your BitBox. Murch, do you want to shill miniscript at all?

Mark Erhardt: I think this is another one of those Legos that we have put into our protocol stack that we will still have to wait a little bit to see the outcomes of. So, you basically get a high-level language that compiles down to a Bitcoin script that you can use to describe output script descriptors. So, when you want to, for example, make a decaying multisig that at first starts out as a 2-of-3 multisig, after a few months becomes a 1-of-3 multisig, you could specify that in a higher level language that’s much more human readable, and it would give you the optimal script expression to put that into your output script. And so, there’s a bunch of wallets now that have built support for this. We are still waiting for a tapscript miniscript support; that’s coming soon, hopefully.

But I assume that as this matures and people are aware of the availability, they will start using this to make more creative solutions, for example, for inheritance planning, or for just key-loss scenarios where as long as you move your funds often enough, it’s always pretty secure with a 2-of-3 multisig or maybe a 3-of-5, but then over time, if you can’t move your coins, it might become easier to move that, so you don’t ever completely lose your access. So, we’ve seen, for example, the Liana wallet by Wizardsardine dig into that concept. I’ve also seen a demo recently by, I think it was AnchorWatch, that has a drag-and-drop editor for miniscript, which I thought was really cool as well.

So, it’ll take some time for all of this to arrive for the user, but when it does, it just makes it a lot easier for developers to be expressive in what they want their scripts to do.

Mike Schmidt: As part of Optech’s effort to educate and provide insights and lessons learned from Bitcoin businesses about Bitcoin tech to other businesses, I’ll give you a sneak preview that we have a miniscript field report that is currently under construction that will go through some lessons learned with rolling out a miniscript heavy piece of software, from some of the pioneer software providers in the miniscript space. So, look forward to that in a few weeks.

Core Lightning 23.08.1

Okay, now actually to the Releases and release candidates section. Core Lightning 23.08.1, which is a maintenance release for CLN, fixes a few bugs. A couple that I saw that might be interesting for folks is that the CLNRest tool that we talked about a few weeks ago, that allows you to interact with your CLN node via REST, now it works on macOS, so I think that maybe wasn’t possible previously. And there were also some minor fixes to the renepay plugin that we covered in a previous podcast as well.

LND v0.17.0-beta.rc4

Next release is LND v0.17.0-beta.rc4. We spoke with roasbeef about this release a bit last week. So, if you’re curious jump back to podcast #268 for that discussion, and listen to it on perhaps half speed, you get more details about this particular release!

Moving on to Notable code and documentation changes, I’ll take the opportunity to solicit from the audience, if you have any questions, feel free to type them in this tweet thread, or you’re also free to request speaker access, and we’ll try to get to your question at the end of our discussion.

Bitcoin Core #26152

First PR is Bitcoin Core #26152. Murch, congrats on getting this merged. This is your PR, so I’ll let you describe the PR and the issue that it resolves.

Mark Erhardt: Yeah, okay, so I’ve been working on this for a while. The idea here is when you use your UTXOs to create a transaction, Bitcoin Core used to always treat all UTXOs as if they were confirmed already for fee purposes. However, when you have an unconfirmed UTXO that is created by an existing transaction with a low feerate and you use that to build a new transaction, you might actually underestimate the necessary fees to achieve the feerate that you intend. So, you’re probably in a situation there then that you create a CPFP, but you didn’t account for the bump fee that you need to elevate the parent transaction to the current feerate. And so, I had a few collaborators that heavily helped with this effort.

A few months ago, we already got the first part of this merged, which is the calculation of how much fees it would cost to elevate or reprioritize an ancestor structure to a certain feerate. And with this PR, we also put it into the wallet, where the wallet will now use it to calculate the effective value of UTXOs. So, when you spend a UTXO that is encumbered by a low-fee parent, then it will already estimate in the effective value how much it has to pay extra to bump the parent to the same feerate. And yeah, so if you, for example, explicitly choose to pay or to use an unconfirmed input in a payment, you will now automatically bump the parent transaction to the same feerate as the new transaction you’re building and hopefully that, for example, fixes issues around when you accidentally have a consolidation transaction and then try to make an urgent payment, or when you otherwise are forced to spend an unconfirmed input that you might not even get into the mempool because it’s below the dynamic minimum mempool feerate. Sorry, it would fix that part maybe by not using the UTXO, because we account for the cost correctly and deprioritize it.

But, yeah, it’s been a big lift, a lot has been written about it. There’s even a video for people that were interested in reviewing it. We did also a Bitcoin Core Code Review Club on it. So, if you’re interested in it, you’ll find a lot of links with more information in the PR.

Mike Schmidt: Excellent, congratulations, Murch. And that sounds like quite a useful feature.

Mark Erhardt: No, it’s a bug fix!

Mike Schmidt: Well, true, yes. You can actually use a CPFP feature correctly; what a novel idea!

Mark Erhardt: Well, we actually don’t have a CPFP RPC yet, but we can build one now.

Bitcoin Core #28414

Mike Schmidt: Soon. Bitcoin Core 28414. As part of the PSBT workflow, there’s a walletprocesspsbt RPC that currently returns Base64-encoded PSBT, along with a boolean indicating if the transaction is, “Complete”. So, you already potentially have a complete and finalized transaction at that point, but you would then have to call the finalizepsbt, even though you already have a finalized transaction. In theory, the walletprocesspsbt could finalize the transaction for you and provide you the final hex of the transaction, and that’s exactly what this PR does. So, with this PR, walletprocesspsbt returns an object and will also include the broadcastable hex string if the transaction is already final. So, that obviously saves users an extra step of calling that finalizepsbt command.

Mark Erhardt: Yeah, and also to explain what walletprocesspsbt does, this is essentially the follow-up or the cousin of signrawtransaction. When you pass a PSBT to the wallet, the wallet will add all the information it has about the parts of the PSBT that concern it, and will sign the parts where it has the private keys. So, if someone has built, for example, a PSBT for an external wallet or for a multiparty process and then passes it around, with walletprocesspsbt, you would be adding the signatures to it. And then of course, if all of the signatures are there, we would also be returning the hex at this point.

Bitcoin Core #28448

Mike Schmidt: Next PR is Bitcoin Core #28448, which deprecates the rpcserialversion configuration parameter. And that is an option that allows users to specify the format of the raw transaction or the block hex serialization. So, this was something that was originally introduced during the transition to v0 of segwit to allow users to continue to access blocks and transactions but without any segwit fields. But given that segwit was activated in 2017, probably most people have upgraded, so this option is now being deprecated. And if you’re somebody who’s still using this, you can re-enable it for now using the -deprecatedrpc option and providing that configuration parameter, the rpcserialversion.

Mark Erhardt: And to explain again here, this is not affecting the Bitcoin Core version that you have deployed. This will be shipped in 0.26, which hopefully we will see in November.

Bitcoin Core #28196

Mike Schmidt: Next PR is also to the Bitcoin Core repository, #28196. This is a PR that is part of BIP324, the encrypted transport protocol that opportunistically encrypts traffic between nodes, and we spoke with Pieter Wuille last week, on podcast #268, about BIP324, so check that out if you missed it, it’s always good to hear Pieter talk. And this PR implements all of what the BIP calls, “The transport layer and the application layer”. And it notes that it does that in a non-exposed way, meaning that you can’t use BIP324 yet, there’s still some things to do to enable that. And this PR also includes, “An extensive fuzz test, which verifies that v2 transports can talk to v2 transports, and v1 transports can talk to v2 transports, and a unit test that exercises a number of unusual scenarios”.

What is left to do to complete the 324, since this adds a lot of meat, but doesn’t actually allow you to use it, we need to, “Actually use the v2 transports for connections”, this is from the PR itself, what’s remaining, “support the NODE_P2P_V2 service flag, retry downgrade to V1 when attempted outbound V2 connections immediately fail”, and then also some P2P functional and unit tests need to still be added in order for BIP324 to be officially enabled. Murch, thoughts?

Mark Erhardt: Yes, so it sounds like it’s essentially one more commit in order to be feature complete, and I’m not sure if the service flag has been picked, but that would be then added and it looks currently like that it might ship with 0.26. So, we might be able to start encrypting all of our traffic with the upcoming release in November.

Mike Schmidt: Amazing, a ton of work, a long time coming, a lot of different people contributing, so it’s good to see that project moving along.

Eclair #2743

Next PR is to Eclair repository, #2743. Eclair nodes currently already provide automatic fee bumping, but this PR adds a fee bumping RPC, called bumpforceclose, that allows users to manually fee bump an anchor output from a channel to use CPFP to bump that commitment transaction. So, a manual way to do, it sounds like, what they are already doing automatically in some cases.

LDK #2176

Next PR is to the LDK repository, #2176. LDK currently stores a bunch of historical estimates of channel liquidity in 8 evenly sized buckets. That allows LDK to somewhat guess the amount of liquidity available in distant channels that it’s attempted to route payments through previously. And the PR notes, “This lacks precision, especially at the edges of channels where liquidity is expected to lie”. So, this PR, “Rips out the existing buckets, those 8 buckets of equal size, and replaces them with 32 unequal sized buckets, which allows LDK to focus their precision at the edges of a channel (where the liquidity is likely to lie and where the precision helps the most)”. And I think there were some figures and numbers around these liquidity numbers in the writeup, but I also cherry-picked some of these quotes from the PR itself.

The PR noted that the change does slow down routing performance a bit, and they’re estimating that it’s a 5% to 8% performance reduction in order to achieve that more accurate granularity of liquidity estimates.

LDK #2413

Next PR is also to the LDK repository, #2413, which also references LDK #2514, which adds support to LDK for sending two blinded payment paths and also for receiving to one-hop paths. And this is part of LDK’s BOLT12 offers efforts. And there is a tracking issue on the LDK repository, which tracks all of the work that they’re doing to support offers, and that is LDK #1970. So, if you’re curious about tracking issues, like I am, go check that out and you can see that a lot of progress has been made there and there’s just a few things left to roll this out to LDK.

Mark Erhardt: I think it’s kind of funny how since you cannot tell how long a blinded payment path instruction is and you can just wrap your own onion around it, if you don’t know what node software is running the receiving node, this is indistinguishable from other nodes that may have already multi-hop blinded path receiving support. So, even just having single-hop receiving support, already adds to the anonymity set or to the feature set of people because they don’t know how many hops it is.

LDK #2371

Mike Schmidt: Last PR is also to LDK, #2371, and this PR is also part of LDK’s BOLT12 implementation. This one adds support for outbound payments for paying a BOLT12 invoice. So, it allows users to one, use an offer to register its intent to pay an invoice with another node. It also then can timeout that payment attempt if a sent offer never results in actually getting the invoice that you wanted. And then it uses the existing LDK code to pay that invoice, including retry mechanisms if the first attempts don’t succeed.

That’s it for #269. Murch, any announcements or anything notable before we wrap up? I don’t see any questions.

Mark Erhardt: I don’t have any announcements.

Mike Schmidt: Well, thanks to Sergi for joining us and thanks always to my co-host, Murch, and for you all taking the time out of your day to listen about Bitcoin technology in our Twitter Space. Cheers.